1. Field of the Invention
The present invention relates to a screen upon which an image (as a picture or information) is projected to display the image on the screen, and more specifically, relates to a display screen which can either selectively display a plurality of images thereon or display a plurality of images in a manner so as to overlay one image on another.
2. Description of the Prior Art
In regard to a display screen for displaying an image, a projection display screen and a transmissive display screen are two possible types. A projection display screen consists of a blank surface and a support structure used with, e.g., a projector. A transmissive display screen consists of a transparent plate a surface of which is formed as a milk-white (translucent-white) surface. For instance, a transmissive display screen is often used as a focusing screen, which serves as an optical element of an single-lens-reflex camera, or a display screen of a liquid crystal display (LCD) or a cathode-ray tube (CRT). The focusing screen is made of a transparent resin plate, a surface of which is roughened (formed to be a light-diffusing surface) to allow a real image of an object to be formed on the roughened surface so that the real image can be viewed due to an optical effect at the roughened surface.
Conventionally, various photographic information are displayed on the focusing screen using a display device so as to be visually readable together with (or separately from) an object image (optical image) formed on the focusing screen. An LCD is generally used as such a display device. Visual information displayed on the LCD is formed at a position on an optical axis which is optically equivalent to the position of the focusing screen by an optical device. In Japanese unexamined patent publication 2000-137268 and Japanese patent publication No. 3539251, a device for displaying required images on the focusing screen using an EL (electroluminescent) display instead of an LCD has been proposed. Specifically, in the former publication, forming an EL display in a peripheral part of the object image forming area (picture area) of a focusing screen and making the EL display indicate required information by making an EL drive circuit drive the EL display makes it possible to view either only the required information or the required information together with an object image displayed on the focusing screen through a viewfinder. Furthermore, the latter publication discloses a system wherein an EL display is overlaid on a focusing screen to display a picture frame on the focusing screen. In the former publication, a system wherein an EL display is provided on a surface of a prism serving as an element of a viewfinder optical system has also been proposed.
In either a conventional projection display screen or a conventional transmissive display screen, the display screen is used to display only one image, and it is difficult to display different images in a manner to overlay (superimpose) one image on another. Although an object image and visual information can be simultaneously displayed by an arrangement in which an EL display is disposed on an image forming surface of either a focusing screen or a prism in the system disclosed in the former publication, an object image and visual information cannot be displayed simultaneously in an superimposed manner because the EL display is disposed outside of the effective field of view, in which an object image is displayed. In addition, images displayed on the focusing screen are limited to object images which are formed via a photographing lens because the EL display displays only visual information. On the other hand, although an image displayed on the EL display is displayed to be superimposed on an object image on the focusing screen in the system disclosed in the latter publication, the system disclosed in the latter publication is the same as that in the former publication in that images displayed by the EL display are limited since the image displayed by the EL display are unchangeable images which are predetermined beforehand. Therefore, in either of the former and latter publications, the degree of flexibility in images displayed on the display screen is low.